Dual antenna



oet. 15,1968

`L J. KULIK DUAL. ANTENNA 2 Sheets-Sheet 1 Fund, @pruzs, v1967 Oct. l5, 1968 J. J. KULIK 3,406,393

4 DUAL ANTENNA Filed April 25, 1967 2 Sheets-Sheet 2 INVENTOR, JOHN J. KUL IK.

ATTORNEYS United IStates Patent O 3,406,393 DUAL ANTENNA John J. Kulik, Belmar, NJ., assignor to the United States of America as represented by the Secretary of the Army Filed Apr. 25, 1967, Ser. No. 634,796 5 Claims. (Cl. 343-7.4)

ABSTRACT F THE DISCLOSURE A dual antenna is described comprising a pair of spaced and nested paraboloids. The inner paraboloid is fed in normal fashion to form a pencil beam. The annular slot formed by the peripheries of the paraboloids is fed from the vertex area to form a split lobe pattern with a null on the focal axis of the paraboloids. A circuit of a search and track radar utilizing this dual antenna is shown.

Specification on the'axis. Such a dual antenna is useful in search and track radars in that both search and track functions can be obtained from the single unitary structure.

It is thus an object of the invention to provide a novel and useful dual parabolicv antenna, each antenna thereof having a different radiation pattern.

A further object of the invention is to provide an antenna structure capable of radiating or receiving simultaneously two signals with different radiation patterns.

These and other objects and advantages of the present invention will become apparent from the following detailed description and drawing in which:

FIGURE 1 is a combination pictorial drawing of an illustrative embodiment of the dual antenna of the present invention and a block diagram of the radar circuitry with which the antenna may be used.

FIGURE 2 is a view of the antenna of FIGURE 1 looking into the aperture thereof and,

FIGURE 3 is a series of waveforms illustrating the operation of the circuitry of FIGURE 1.

In FIGURE 1, the dual antenna 8 therein is formed by nesting two spaced paraboloids or dishes 5 and 6. The forward or `inner paraboloid 6 functions as a conventional parabolic antenna and in the present example is shown as a Cassegrainian type with a sub-reflector 9 within the aperture thereof, supported by struts 11. 'Ihe sub-reflector is illuminated by the energy emitted from the open-ended round waveguide 17 which projects through the vertices of both paraboloids and is conductively connected to the inner paraboloid 6. The parabolic portion of the antenna is fed from rst d-uplexer 21, which is connected to the opposite end of round waveguide 17. The outer surface of waveguide 17 forms the inner conductor of a coaxial line 19` which feeds the annular slot defined by the space 7 between the two nested paraboloids. The coaxial line is terminated at its end remote from the paraboloids in a short circuit 18. One quarter wavelength away from this short a second coaxial line 20, mounted at iight angles to the line 19, has its center conductor connected to the outer surface of waveguide 17 and its outer conductor connected to the outer conductor of the line 19. Thus, a microwave signal 3,406,393 Patented Oct. 15, 1968 ice launched down line 20 from the second duplexer 23 will excite the TEM mode in the line 19. The arrows 13l illustrate the electric vectors in the line 19` and in the annular slot antenna. As seen in FIGURE 2, the vectors within the slot are all radially symmetrical about the center of the antenna aperture or focal axis and therefore will give rise toa radially symmetrical radiation pattern with a null on the axis. This pattern is illustrated in a single plane by the diagram S1 of FIGURE 1. The actual three dimensional pattern is a ligure of revolution obtained by rotating the pattern 51 about the boresight or null axis 52. The radiation pattern of the forward or inner paraboloid is shown at 53.

As seen in FIGURE 2 the diameter D2 of the forwarded paraboloid 6 is made less than the diameter D1 `of the rear paraboloid 5 and this ditference in diameters controls the annular slot width. If the two paraboloids have the same curvature at corresponding points the distance between them will be uniform at each point. This is the case illustrated in the drawings. It may be desirable in certain applications to provide a spacing which flares or diverges from the vertex toward the periphery. This will result in a wider annular slot. Such a structure can be accomplished by making the focal length of the rear paraboloid greater than that of the forward paraboloid. Also, in FIGURE 1 the peripheries of the two paraboloids are shown lying in the same plane, a vertical plane if the boresight axis 52 is directed horizontally. Here again this feature is not essential to the practice of the invention and the paraboloid peripheries may lie in different, but parallel planes.

The circuitry of FIGURE 1 and the waveforms of FIGURE 3 show how the dual antenna 8 may be used as part of a search and track radar system. In such a system, the radar is first operated in the search mode, usually employing a pencil type beam as shown at 53 in FIGURE 1. When a target is thus detected, the system is switched over to the tracking mode in which the antenna continually tracks the moving target by means of a servo system. The circuitry of FIGURE 1 includes a pulse modulated microwave transmitter 25, the output of which is alternately applied to first and second duplexers 21 and 23 by means of switch 26. Thus the transmitter pulses are alternated between the two duplexers and hence the slot and parabolic antennas are energized by altenate transmitter pulses. Target echoes from the parabolic antenna are applied to rst receiver 27 via iirst duplexer 21 and echoes from the slot antenna to the second receiver 35 via the second duplexer 23. The demodulated video target pulse trains of the two receivers are combined in adder 29. The adder output is applied to integrator 31, the output of which operates trigger 43, which may be of the Schmitt type which will produce an output if the integrator output exceeds a certain threshold voltage and no output if the integrator output falls below the threshold. The trigger circuit output is applied to the control input of transmission gate 39. The adder output is also applied to boxcar circuit 33. This circuit holds the peak amplitude of each video pulse applied thereto until the next pulse arrives, at which time the process is repeated. The modulation detector senses the maximum amplitude of the alternating component of the boxcar circuit output and controls the servomechanism 41 in accordance therewith. The servomechanism output is applied to the antenna search and track mechanism through gate 39. The operation of the system is as follows: The mechanism 37 controls the antenna movement via mechanical connection 38. In the search mode this mechanism systematically scans an area of space for targets. If a target is picked up in the beam of either antenna, the resulting target echoes will build up the voltage on integrator 31 to such an extent that trigger 43 will be operated, thereby closing normally .open gate. .'3-9. and.,

In this condition .both antennas will receive target echoes,

but those of the receiver 27 will be larger than those of the receiver 35 since in the vicinity of the boresight axis, the response or gain of the parabolic antenna is higher than that of the slot antenna. Since the adder interleaves they two'receiver video pulse trains intime, the boxcar out-put will be alternately high and low. This' situation is illustrated in FIGURE 3. The Waveform` a therein illustrates the output of the slot antenna receiver 35 and the waveform b the output of the parabolic antenna receiver 27. The waveform c is the output of the boxcar circuit. Under the control of the circuit 41, the antenna will be moved by trial and error until the modulation of the boxcar circuit is a maximum. This will occur when the target is on the boresight axis, since the parabolic antenna output is a maximum there and the slot antenna output zero. The servomechanism then moves the antenna in such a direction as to maintain this condition.

While a specic embodiment of the invention has been shown and described, obvious modifications can be made thereto without departing from the spirit of the invention. For example, instead of the Cassegrainian type feed, the parabolic antenna may be fed from a horn at the focus thereof, the horn being an extension of the feed waveguide. Accordingly, the invention should be limited only by the scope of the appended claims.

What is claimed is:

1. A dual antenna comprising a pair of spaced paraboloids, one of said paraboloids being nested inside of the other paraboloid, the focal axis of the paraboloids being collinear, said one of said paraboloids being fed in normal fashion, resulting in a parabolic antenna with formed by the peripheries of said paraboloids in such a manner as to form a radially symmetrical beam about said focal axis with a null on said axis.

2. The dual antenna of claim 1 wherein said one of said paraboloids isfed Abymeans of ka hollow waveguide which projectsthrough Ythe vertices of .both-paraboloids from the rear and is conductively connected to .said one of said parabolods,-and said annular slot is fed by means of a coaxial line whichY communicates with the space between said paraboloids, Atheinner conductor of said coaxial linebeing the outer surface of said hollow waveguide.

3. The dual antenna of claim- 1 wherein said hollow waveguide vhas a circular cross-section. l 4. The structure of claim 1, further including transmitter, means to alternately apply the output of said transmitter to said parabolic antenna andto said slot antenna, and means responsive to the amplitudes of target echoes received by said parabolic and slot antennas to move said structure in such direction that the boresight axis of said paraboloids remains fixed on the source of said target echoes.

5. A dual microwave antenna comprising a pair of nested and spaced paraboloids, means to feed the inner paraboloid from tthe focal area thereof to form a pencil beam, and means to feed the annular slot defined by the peripheries of said nested and spaced paraboloids in such a manner as to form a split-lobed beam with a null on the focal or boresight axis of said paraboloids.

References Cited UNITED STATES PATENTS 3,229,283 1/1966 Hefter et al. 343-16 X RODNEY D. BENNETT, Primary Examiner. T. H. TUBBESING, Assistant Examiner.

a radar Y 

